An ideal ungrounded power distribution system would have an infinite impedance between all conductors and ground so that no current would flow to ground. In the ideal case, if a low impedance path were connected from one conductor to ground, no current would flow through the low impedance path to ground. There is no return path from ground to a power conductor, therefore no current can flow between the power system and ground.
However, in reality the impedances between the conductors and ground are actually finite, and are defined as fault impedances. In the realistic case, if a low impedance path is established from one conductor to ground by making a low impedance test connection therebetween, current can flow to ground. A return path is provided through the fault connections. The current that would pass through the low impedance test connection between a power conductor and ground is limited by fault impedances to ground of the other power conductors. Thus, a measure of the isolation from ground of an ungrounded power system is the current flowing through the test connection. The maximum current which can flow through the test connection is known as the hazard current. The hazard current is related to the minimum fault impedance of the system.
In a two conductor power distribution system, each conductor has an associated fault impedance. The low impedance test connection can be made between either conductor and ground. The hazard current is the largest current than will flow through the test connection in either one of these positions.
Fault impedance is defined in Underwriter's Laboratories Standard 1022 as, "an impedance that may consist of balanced or unbalanced resistive and capacitive, or combined resistive/capacitive loads to either or both isolated conductors". Balanced faults are equivalent fault impedances on both conductors. Similar type fault impedances of different magnitude are unbalanced faults. Dissimilar type fault impedances are hybrid faults. A combined resistive and capacitive fault impedance on the same conductor is denoted as an RC fault.
The total hazard current is measured with all devices connected, including line isolation monitors. The entire electrical system is known as an impedance network. Components of the total hazard current are the monitor hazard current, and the fault hazard current, that portion of the total hazard current resulting when all devices, except the line isolation monitors are connected.
A line isolation monitor (LIM) is used to supervise the total hazard current of an ungrounded power distribution system. In doing so, the monitor is required to be equally sensitive to any combination of fault impedances and to indicate on a scale the total hazard current and issue an alarm if this value exceeds a predetermined limit.
The monitor according to the invention is connected to the power conductors of an ungrounded power distribution system and ground reference, and will measure the maximum admittance to ground of any power conductor at the line frequency. Admittance is defined as the reciprocal of impedance. Therefore, the maximum admittance is proportional to the minimum impedance. Thus, the total hazard current is proportional to the maximum fault admittance. The characteristics of the line voltage across the ungrounded power distribution system are considered constant. Admittance is a complex vector value that may have frequency dependent components. Admittance can be measured directly by Ohm's Law at the frequency of interest by measuring the voltage response to a known current signal or conversely the current response to a known voltage signal. These responses, at the frequency of interest, if measured on an activated power system, could not be distinguished from the voltage across the fault admittance due to line voltage.